Silver alloy excelling in performance of reflectance maintenance

ABSTRACT

The present invention is a silver alloy including silver as the main component and at least one rare-earth element as a first dopant element, and excellent in reflectance maintenance property. In the present invention, the first dopant element is preferably samarium, neodymium, lanthanum, cerium, ytterbium, terbium, dysprosium, holmium, erbium, thulium, europium, and gadolinium. Also, in the present invention, the silver alloy preferably includes, as a second dopant element, at least one element selected from copper, manganese, silicon, chromium, nickel, cobalt, yttrium, iron, scandium, zirconium, niobium, molybdenum, tantalum, tungsten, platinum, gold, rhodium, iridium, palladium, indium, tin, lead, aluminum, calcium, gallium, bismuth, antimony, strontium, hafnium, and germanium.

TECHNICAL FIELD

The present invention relates to a silver alloy suitable as a materialfor forming reflective films provided on optical recording media,displays and the likes. Particularly, the present invention relates to asilver alloy for reflective films capable of maintaining the reflectancethereof even in long-term use.

BACKGROUND ART

Silver is recognized as a most preferable material as a material forreflective films to be used in optical recording media, displays and thelike. This is because silver is high in reflectance, and additionally,lower in price than gold that is also high in reflectance. In the fieldof optical recording media in these years, transition torecordable/rewritable media (CD-R/RW, DVD-R/RW/RAM) takes place, andexpanding importance of optical recording media attaches importance tosilver that is high in reflectance and relatively low in cost as acentral material for use in reflective films.

On the other hand, silver involves a problem such that silver is poor incorrosion resistance and is degraded in reflectance by discoloration toblack through corrosion. The root causes for the corrosion of reflectivefilms may vary with products (recording media, displays and the like) towhich the reflective films are applied, in such a way that, for example,reflective films of displays suffer from a fear that they are corrodeddue to the atmospheric humidity or the like.

A reflective layer of an optical recording medium also suffers from afear of corrosion due to the atmosphere such as the air, andfurthermore, requires a consideration of corrosion caused by the effectsof the other constituent layers (the recording layer, dielectric layerand the like) in contact with the reflective layer in the recordingmedium. For example, a recordable optical disk (CD-R, DVD-R, DVD+R orthe like) has a structure in which an organic azo or cyanine dye ink iscoated on a polycarbonate substrate and dried to form a recording layer,a reflective layer is formed on the surface of the recording layer, andthe recording layer with the reflective layer is bonded to anotherpolycarbonate substrate with an ultraviolet cure adhesive. In this case,the organic dye ink in the recording layer and the ultraviolet cureadhesive, both in contact with the reflective layer, contain sulfur as acomponent thereof or as an impurity, and accordingly, the reflectivelayer is exposed to a fear of corrosion in the course of use thereofbecause silver is poor in resistance to sulfidation. On the other hand,a rewritable optical disk (CD-RW, DVD-RW, DVD+RW, DVD-RAM or the like)has a structure in which a derivative layer, a recording layer, adielectric layer and a reflective layer are laminated in such acondition that the reflective layer is in contact with the dielectriclayer. Various materials are available for the dielectric layer, but inthese years, materials to which zinc sulfide (ZnS) is applied are beingused (sometimes, for the purpose of controlling heat reserve, a materialmade of zinc sulfide doped with silicon oxide, namely, ZnS-20 mol % SiO₂or the like being applied). Consequently, also in this case, there is afear of corrosion due to a sulfide. As described above, the reflectivelayer of an optical recording medium is in such an environment that thereflective layer is in contact with a layer containing sulfur or asulfide, irrespective as to whether the optical recording medium isrecordable or rewritable in type; thus, the resistance to sulfidationcomes to be more significant than the corrosion caused by the atmosphereinvolving humidity or the like, and there is a fear that the reflectanceof the reflective layer is degraded by the long-term use of therecording medium.

Additionally, there is a problem that a reflective film made of silveris also thermally degraded in reflectance. The mechanism involved in thereflectance degradation due to heating is not yet elucidated, but it hasbeen verified that heating of a thin silver film causes a phenomenon inwhich local agglomeration occurs in the thin film so as to expose theunderlayer. Accordingly, the reflective film of an optical recordingmedium, a plasma display or the like is required to have a heatresistance because it possibly undergoes heating.

For the purpose of coping with the reflectance degradation of thereflective film as described above, there have conventionally beendeveloped silver alloys, for use in reflective films, that are improvedin corrosion resistance and heat resistance while the reflectance beingsecured. These alloys mostly include silver as the main component, andare doped with one or more of various elements as dopant elements;examples of such disclosed alloys include, for example, an alloy inwhich silver is doped with 0.5 to 10 atomic % of ruthenium and 0.1 to 10atomic % of aluminum, and another alloy in which silver is doped with0.5 to 4.9 atomic % of palladium. It has been disclosed that thesesilver alloys are satisfactory in corrosion resistance, can maintain thereflectance in service environment, and consequently, suitable forreflective films (for the details of the conventional art, see PatentDocuments 1 and 2).

Patent Document 1: Japanese Patent Laid-Open No. Hei 11-134715

Patent Document 2: Japanese Patent Laid-Open No. 2000-109943

As for the above described silver alloys, improvements of the corrosionresistance and heat resistance have been developed to some extents.However, even these silver alloys are not absolutely free fromdeterioration in service environment. Additionally, the reflectance isnot completely guaranteed against reflectance degradation, and materialsthat can maintain the reflectance at a higher level are desired.

In the field of optical recording devices, at present, red semiconductorlasers (wavelength: 650 nm) are applied as recording light sources, butblue lasers (wavelength: 405 nm) have almost seen their way clear topractical use. Application of the blue laser can ensure a memorycapacity 5 to 6 times as large as the memory capacity of an opticalrecording device available at present, so that optical recording devicesincorporating blue lasers applied thereto will conceivably form themainstream of the next-generation optical recording devices. In thisconnection, according to the present inventors, it has been verifiedthat the reflectance of a reflective film varies with the wavelength ofthe incident laser light; in particular, it has been verified thatshorter-wavelength laser irradiation degrades the reflectanceirrespective as to whether corrosion occurs or not, and makes the extentof the reflectance degradation due to corrosion frequently larger thanthe longer-wavelength laser irradiation. Consequently, for the purposeof producing recording media adaptable to the development of the futurerecording light sources, it is desired to develop a material that has ahigh reflectance even for the laser irradiation in a shorter wavelengthregion, and furthermore, can maintain the reflectance within a range ofpractical use.

The present invention has been achieved on the above describedbackground, and is aimed at providing a material that is a silver alloyto form a reflective film of an optical recording medium or the like,and is workable without degrading the reflectance even in a long-termuse. Additionally, the present invention provides a material that has ahigh reflectance even for a short wavelength laser light.

DISCLOSURE OF THE INVENTION

In order to solve the above described problems, the present inventorsselected appropriate dopant elements with silver as the main componentsimilarly to the conventional art. Consequently, the present inventorshave come up with the present invention by discovering that the additionof a rare-earth element, as a dopant element is effective in maintainingthe reflectance and is useful for improving the heat resistance, themoisture resistance or the resistance to sulfidation.

The present invention is a silver alloy for use in a reflective film,including silver as a main element and at least one rare-earth elementas a first dopant element.

Here, the rare-earth element as the first dopant element is selecteddepending on which of the properties of the thin film is considered asimportant. According to the present inventors, the rare-earth elementsuseful for improving the performances of the silver alloy thin film aredysprosium, thulium, terbium, gadolinium, erbium, neodymium, holmium,praseodymium, samarium, lanthanum, cerium, ytterbium, and europium.Inclusion of at least one of these metal elements can lead to a silverthin film useful for maintaining the high reflectance.

According to the investigations carried out by the present inventors, ithas been verified that a silver alloy, doped with dysprosium or thuliumamong the above described elements, can maintain the various propertiesrequired for a reflective film at specially high levels. Such asilver-dysprosium alloy and such a silver-thulium alloy are excellentboth in corrosion resistance and in heat resistance, and are suitablenot only for the reflective layer for use in an optical recording mediumbut also for the reflective film for use in a display.

Further, in the present invention, the silver alloy is preferably analloy doped with at least one element, as a second dopant element,selected from gallium, platinum, palladium, magnesium, zinc, nickel,molybdenum, gold, aluminum, copper, cobalt, tin, titanium, bismuth,manganese, scandium, yttrium, silicon, chromium, iron, zirconium,niobium, tantalum, tungsten, rhodium, iridium, indium, lead, aluminum,calcium, antimony, strontium, hafnium, germanium, boron and strontium.These elements have effects to improve, in cooperation with the firstdopant element, the resistance to sulfidation, moisture resistance andheat resistance, and work in combination with the first dopant elements.

In particular, a silver alloy doped with a second dopant element ofgallium, platinum or palladium is a preferable alloy because such analloy can effectively suppress the agglomeration phenomenon whichpossibly occur in a thin film material in a humidified environment.

Additionally, the dopant element concentration, namely, total of theconcentration(s) of the first dopant element(s) and the concentration(s)of the second dopant element(s) is preferably 0.01 to 5.0 atomic %. Whenthe addition amount is less than 0.01 atomic %, no effect of thereflectance maintenance is found. When the dopant element concentrationexceeds 5.0 atomic %, the reflectance degradation becomes largedepending on the service environment and the incident laser lightwavelength, and it becomes impossible to ensure the reflectance. Theconcentration is particularly preferably 0.01 to 3.0 atomic %, becausethis range of concentration can maintain the reflectance at a higherlevel irrespective of the service environment and the laser lightwavelength.

The above-described silver alloy, according to the present invention, asthe material for use in a reflective film, can be produced by the meltcasting method and the sintering method. The production based on themelt casting method involves no particular difficulties, and can producethe alloy by means of a general method in which individual raw materialsare weighed out, mixed by melting, and the mixture is subjected tocasting. The sintering method also involves no particular difficulties,and can produce the alloy by means of a general method in whichindividual raw materials are weighed out, and subjected to sintering.

The silver alloy according to the present invention has propertiesfavorable for a reflective film, and suppresses the reflectancedegradation in the course of use. As described below, the silver alloyaccording to the present invention exhibits a more satisfactoryreflectance and a more satisfactory maintenance of the reflectance thanconventional materials for use in reflective films, even underirradiation with a short wavelength laser light. As described above, thesputtering method is generally applied to the production of reflectivefilms of optical recording media and the like. Accordingly, sputteringtargets made of the silver alloy according to the present invention canserve to produce optical recording media, displays and the like eachprovided with a reflective film having satisfactory properties.

As described above, according to the present invention, a reflectivefilm that is less degraded in reflectance even under long-term use canbe produced, and various devices to which reflective films are applied,such as optical recording media, displays and the like, can be made tohave a long operation life. The silver alloy according to the presentinvention also exhibits a more satisfactory reflectance and a moresatisfactory maintenance of the reflectance than conventional materialsfor use in reflective films, even under irradiation with a shortwavelength laser light. Accordingly, the silver alloy according to thepresent invention is compatible with recording media for use in opticalrecording devices each having a short wavelength laser as the lightsource, such recording devices being anticipated to form the mainstreamof such devices.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of the present invention isdescribed together with comparative examples. Here, binary and ternarysilver alloys having various compositions with Ag as the main componentwere produced; targets were produced from these alloys, and thin filmswere formed therefrom by means of a sputtering method. These thin filmswere subjected to corrosion tests (accelerated tests) in variousenvironments, and the reflectance variations as observed after thecorrosion tests were examined.

In the production of each of the silver alloys, individual metals wereweighed out so as to give predetermined concentrations, mixed by meltingin a high frequency melting furnace to yield an alloy. The molten alloythus obtained was cast into a casting mold to be solidified to yield aningot, the ingot was forged, rolled and heat treated, and thereaftershaped into a sputtering target.

In the thin film production, a substrate (borosilicate glass) and atarget were placed in a sputtering apparatus, the interior of theapparatus was evacuated to a vacuum of 5.0×10⁻³ Pa, and then argon gaswas introduced into the interior of the apparatus so as to give apressure of 5.0×10⁻¹ Pa. Sputtering conditions were such that filmformation was carried out with a direct current power of 4 kW for 8seconds to attain a film thickness of 1200 Å. It is to be noted that thefilm thickness distribution fell within ±10%.

The thin film products were evaluated with respect to heat resistanceand moisture resistance. These properties were evaluated as follows: athin film was exposed to each corresponding environment and thereflectance of the thin film after each test was measured at a varyingwavelength in a spectrophotometer; and the observed reflectances atdifferent wavelengths were compared differentially with thecorresponding reflectances of silver as the reference levels, which weremeasured immediately after film formation.

The heating test to examine the heat resistance of a thin film wascarried out as follows: the thin film was placed on a hot plate, heatedin an atmosphere at 250° C. for 1 hour, and the reflectance afterheating was evaluated. The humidification test to examine the moistureresistance of a thin film was carried out as follows: the thin film wasexposed to an atmosphere of a temperature of 100° C. and a humidity of100%, and the reflectance after humidification was evaluated; theexposure time was varied to take two periods, namely, 24 hours(humidification test I) and 100 hours (humidification test II). Theresults of these corrosion tests are shown in Tables 1 to 3. Thereflectances shown in these tables are relative values based on thereflectances of silver immediately after film formation which are set at100. The measured reflectance values are the values measured at thewavelengths of 400 nm, 550 nm and 650 nm (respectively corresponding tothe wavelengths of blue, yellow and red lasers). It is to be noted thateach of these tables includes, for comparison, the test results obtainedfor a thin film produced from a target made of pure silver. TABLE 1Incident light wavelength: 400 nm Reflectance Immediately Humid- Sampleafter Heating Humidification ification composition (at %) depositiontest test I test II Ag—0.5Sm 91.6 89.7 70.3 68.3 Ag—0.8Nd 80.6 44.5 89.678.4 Ag—0.6Yb 99.3 80.1 58.6 58.7 Ag—0.7Eu 83.4 85.0 76.4 77.0Ag—0.4Sm—0.6Ga 102.4 94.1 71.6 15.0 Ag—0.4Sm—0.5In 104.3 91.1 80.7 35.7Ag—0.7Nd—0.7Cu 89.0 87.8 71.9 70.8 Ag—0.5Nd—0.5In 92.8 70.8 83.5 83.0Ag—0.5Gd—0.5In 92.4 69.9 84.6 81.6 Ag—0.5Tb—0.5In 91.3 66.8 92.5 62.5Ag—0.5Ho—0.5Cu 94.5 84.1 80.3 58.5 Ag—0.5Er—0.5Cu 94.5 87.1 82.9 63.3Ag—0.5Tm—0.5Cu 90.7 81.9 83.1 59.9 Ag—0.5Sm—0.5Cu 92.5 81.1 79.0 54.7Ag—0.5Gd—0.5Cu 93.2 86.1 81.3 60.1 Ag (for comparison) 102.3 1.6 45.937.5

TABLE 2 Incident light wavelength: 550 nm Reflectance Immediately Humid-Sample after Heating Humidification ification composition (at %)deposition test test I test II Ag—0.5Sm 96.9 97.5 94.2 95.5 Ag—0.8Nd99.7 63.2 97.5 93.5 Ag—0.6Yb 100.5 90.4 94.8 90.5 Ag—0.7Eu 99.8 96.594.6 94.5 Ag—0.4Sm—0.6Ga 100.3 96.1 88.7 21.1 Ag—0.4Sm—0.5In 99.7 92.885.5 40.5 Ag—0.7Nd—0.7Cu 95.7 95.5 85.2 84.0 Ag—0.5Nd—0.5In 96.7 82.689.3 89.7 Ag—0.5Gd—0.5In 97.3 81.3 92.3 91.4 Ag—0.5Tb—0.5In 96.4 82.292.0 71.2 Ag—0.5Ho—0.5Cu 98.9 94.6 90.2 67.2 Ag—0.5Er—0.5Cu 99.0 95.592.4 71.1 Ag—0.5Tm—0.5Cu 96.9 93.9 95.3 69.0 Ag—0.5Sm—0.5Cu 97.3 93.591.1 67.0 Ag—0.5Gd—0.5Cu 98.7 96.1 92.5 68.8 Ag (for comparison) 100.81.5 78.4 67.8

TABLE 3 Incident light wavelength: 650 nm Reflectance Immediately Humid-Sample after Heating Humidification ification composition (at %)deposition test test I test II Ag—0.5Sm 98.1 98.9 97.2 97.3 Ag—0.8Nd99.9 70.5 98.0 95.1 Ag—0.6Yb 100.6 91.6 96.9 92.2 Ag—0.7Eu 100.2 97.896.0 95.3 Ag—0.4Sm—0.6Ga 100.9 98.8 89.5 22.9 Ag—0.4Sm—0.5In 99.1 92.986.8 42.0 Ag—0.7Nd—0.7Cu 96.9 97.3 87.0 86.3 Ag—0.5Nd—0.5In 97.7 84.291.8 92.3 Ag—0.5Gd—0.5In 97.7 83.2 94.2 93.4 Ag—0.5Tb—0.5In 97.3 86.394.1 73.2 Ag—0.5Ho—0.5Cu 99.2 96.6 91.2 67.8 Ag—0.5Er—0.5Cu 99.4 97.293.2 71.3 Ag—0.5Tm—0.5Cu 97.7 96.2 96.5 69.6 Ag—0.5Sm—0.5Cu 98.0 95.892.6 68.7 Ag—0.5Gd—0.5Cu 99.1 97.5 93.3 69.4 Ag (for comparison) 100.61.6 86.0 78.6

As can be seen from these results, the thin films produced from thesilver alloys according to the present example gave the reflectancevalues higher than those of silver to verify the effects of improvingthe heat resistance and the moisture resistance. As a general tendency,with decreasing incident light wavelength, the reflectance was lowered.

Next, part of the produced thin films were subjected to a sulfidationtest for the purpose of evaluating the resistance to sulfidation in sucha way that the reflectance values after the test were evaluated. In thesulfidation test, each thin film was soaked in a 0.01% aqueous solution(temperature: 25° C.) of sodium sulfide for 1 hour. The results thusobtained are shown in Table 4; from these test results, it has been ableto be verified that in all the wavelength regions, the resistance tosulfidation tends to be improved for the alloy thin films according tothe present embodiment. TABLE 4 Reflectance 400 nm 550 nm 650 nmImmediately Immediately Immediately Sample composition after After afterAfter after (at %) deposition test deposition test deposition After testAg—0.5Sm 91.6 37.0 96.9 45.9 98.1 58.4 Ag—0.4Sm—0.5In 104.3 52.4 99.768.0 99.1 76.7 Ag—0.5Nd—0.5In 92.8 44.0 96.7 68.2 97.7 75.4Ag—0.5Gd—0.5In 92.4 53.0 97.3 77.5 97.7 85.2 Ag—0.5Dy—0.5In 92.7 50.297.9 72.4 98.4 81.0 Ag—0.5Er—0.5In 95.2 47.8 98.7 68.6 99.1 78.4Ag—0.5Tb—0.5In 91.3 53.5 96.4 73.4 97.7 80.8 Ag (for comparison) 102.331.3 100.8 39.6 100.6 53.5

Next, the applicability as a practical reflective film was evaluated foreach silver alloy film according to the present invention. Actually,this evaluation was carried out on the basis of two evaluation methods:one was a convenient simulated evaluation method that was devised by thepresent inventors and the other was an evaluation method in which anoptical recording medium was actually produced and the performance(presence or absence of error) thereof was examined. The reason why theformer simulated evaluation was carried out in the present invention isas follows: in an optical recording medium taken as an example, theoptical recording medium is composed of a large number of layersincluding, in addition to the reflective layer, a substrate and arecording layer, and including, depending on the type of the medium, adielectric layer, a heat release layer and the like; and for the purposeof evaluating the adaptability of the reflective layer of the opticalrecording medium, it is necessary that these constituent layers areformed on the substrate to produce the recording medium, and then therecording medium is evaluated, and this is troublesome.

For optical recording media in practical use, it is relatively easy toproduce such media for an evaluation purpose. However, high-densitylarge-capacity recording media under development for practical use infuture, such as HD-DVD disks and Blu-lay disks, become more complex instructure and have reflective layers to work as semitransparentreflective layers that are anticipated to be used as further thinnerfilms (the film thickness of the reflective layer of an opticalrecording medium currently used being around 1000 to 1200 Å, but thereflective layer of such a next generation optical recording mediumbeing developed for a film thickness of 200 Å or less). As for suchmedia to be complex as described above, it is difficult to actuallyproduce and evaluate them.

Consequently, for the purpose of evaluating the adaptability of a thinfilm as a reflective film without producing an optical recording medium,the present inventors has adopted a method, as a convenient method, inwhich such a film is exposed in a predetermined environment, andthereafter the surface appearance of the film is observed to evaluatethe adaptability of the thin film to practical use.

In this method, the humidification test is carried out as follows:first, a formed thin film is allowed to stand (for 20 to 30 minutes) ina cooling atmosphere set at a temperature (preferably 10° C.) lower thanroom temperature to be sufficiently cooled together with the substrate;the thus cooled thin film is exposed in a humidification environment;and the thin film is taken out and dried, and thereafter a surfaceappearance of the thin film is observed. The humidification environmentis preferably an atmosphere at a temperature of 100° C. and with ahumidity of 100%, and the exposure time in this case is preferably setat 20 minutes. In the heating test, a formed thin film is directlyplaced together with the substrate in a heating environment; the heatingenvironment is preferably an air atmosphere at 250° C., and the exposuretime in this case is preferably set at 60 minutes.

The observation of the surface appearance is conducted to evaluate theevaluation of the degree of generation of silver agglomerates(hereinafter referred to as black spots) occurring as black spots on thethin film surface after having been exposed in each environment. This isbased on the consideration that: as a form of deterioration of a heatedsilver alloy film, silver agglomerates are locally generated on thesurface of the film; silver agglomerates conceivably affect theproperties of a reflective film; and accordingly, the evaluation of thegeneration degree of the black spots allows the adaptability as areflective film to be determined. In this evaluation, the black spots tobe counted are preferably those each having a size of 1 to 10 μm. Suchclear definition of the spots to be evaluated allows the evaluation tobe facilitated. Such a size as described above matches the size of marksto be used for record reproduction in an optical recording medium.

The evaluation of the generation degree of the black spots may becarried out, for example, by taking a photo of the surface of the thinfilm, and by subjecting the photo to image processing to derive the rateof the area of the black spots. As a more convenient method, there isone in which evaluation is carried out by taking as a reference thesurface condition (in this case, almost no black spots being generated)of a silver thin film immediately after formation thereof and byassessing the surface condition of a thin film after heating, relativelyto the reference, through classification into a few levels.

In the present embodiment, the humidification environment was set to bean atmosphere at a temperature of 100° C. and with a humidity of 100%,with the exposure time set at 20 minutes. In the simulated evaluation ofa thin film in the present embodiment, different type of 1200 Å silveralloy thin films was produced, and each thin film was cooled andthereafter exposed in the above described humidification environment tobe subsequently subjected to optical microscopic observation of thesurface appearance thereof. The surface condition of a silver thin filmimmediately after the production thereof was taken as a reference to begraded as “level 1,” and the surface condition evaluation was carriedout in terms of 5 grades in the degrading order of the surface conditionfrom level 1 (in the order of increasing number of the black spots) insuch a way that the properties of the films were assessed throughclassification into level to level 1 to 5. The results thus obtained areshown in Table 5. TABLE 5 Film thickness: 1200 Å Samples (at %) LevelBinary alloy Ternary alloy 1 Ag—1.0Tm Ag—1.0Dy 2 Ag—1.0Tb Ag—1.0GdAg—1.0Ni—1.0Ga Ag—1.0Er 3 Ag—1.0Nd Ag—1.0Ho 4 Ag—1.0Pr 5 Ag—1.0SmAg—1.0Yb Ag—0.5Sm—0.8In Ag—1.0La Ag—1.0Ce Ag—1.0EuLevel 1 refers to the surface condition of silver immediately afterdeposition.

As a result of the simulated test, it is inferred that excellent, inproperties, among the binary alloys are those containing thulium ordysprosium, and excellent, in properties, among the ternary alloys isthat containing gallium as the second dopant element.

Next, DVD-R media having as a reflective layer thin films made of thesilver alloys according to the present invention were actually producedand were evaluated for the properties as the reflective film of anoptical recording medium. In this test, there were used as substratespolycarbonate substrates (120 mm in diameter, 0.6 mm in plate thickness,0.17 μm in groove depth, 0.3 μm in groove width, and 0.74 μm in groovepitch) with preformatted pattern formed thereon, produced with aninjection molding machine equipped with a stamper. On the upper side ofeach substrate, a metal-containing azo recording ink was coated by wayof spin coating and dried, and thereafter, a reflective film was formedso as to have a film thickness of 1200 Å with a sputtering targetproduced in the present embodiment. To each substrate, a polycarbonatesubstrate the same in size as the substrate concerned was bonded with anultraviolet curable resin to produce a DVD-R medium.

Then, with an optical disk evaluation apparatus (optical disk evaluationapparatus, ODU1000, manufactured by Pulstec Industrial Co., Ltd.), thejitter values, P18 errors and PO errors of the thus produced DVD-R mediain the initial condition subsequent to the production thereof weremeasured, and were verified to fall within the ranges of the DVDspecifications. After the verification, the DVD-R media were subjectedto an accelerated environment test in which the DVD-R media were exposedin an environment at a temperature of 80° C. and with a relativehumidity of 85%, and the DVD-R media after the accelerated environmenttest were subjected to the measurement of the individual values by meansof the evaluation apparatus.

FIGS. 1 to 3 show relations between humidification time and jittervalue, P18 error value and PO error, respectively, which were allmeasured in this test. These figures also show the results obtained byapplying the same tests to a DVD-R medium having a reflective film madeof pure silver and commercially available DVD-R media.

As can be seen from these figures, it has been verified that therecording media each provided with a reflective film made of a silveralloy according to the present invention each clear the specificationsfor the individual values, and have a long-term stability, even after along time humidification. On the contrary, the recording medium providedwith a reflective film made of pure silver failed to be recognized bythe recording device and became unusable after humidification for 150hours. It was also verified that the jitter values of the commerciallyavailable products exceeded the specification, and although the errorvalues of these commercial products were able to clear thespecifications, the properties of these commercial products wereinferior to those of the recording media according to the presentembodiment.

Additionally, Table 6 shows results obtained as follows: DVD-R mediaeach provided with a reflective film made of a silver alloy different incomposition from those examined above were produced; the DVD-R mediathus produced were exposed in a humidification environment in the samemanner as described above; and thereafter, the results shown in Table 6,namely, the jitter values, PI8 errors and PO errors thereof weremeasured. TABLE 6 Jitter value PI8 error PO error After After Afterexposure exposure exposure to to to Initially humidity Initiallyhumidity Initially humidity Examples Au—1.0Nd—1.0Ga 7.7 7.8 48 241 5 24Ag—1.0Ho—1.0Al 7.7 7.8 51 257 6 25 Ag—1.0Pr—0.3Ni 7.7 7.8 53 246 5 21Ag—1.0Yb—0.2Co 7.8 7.9 97 261 8 42 Ag—0.5Sm—1.0Sn 7.8 7.9 113 277 9 48Comparative Ag 8.2 11.7 207 299 25 83 Examples Commercial product 8.610.7 224 305 31 73 from Company A Commercial product 8.3 ≧11.9 185 ≧31845 ≧104 from Company B Specification 8.00 or less 280 or less 182 orless

Also from this table, it was verified that the individual values of therecording media each provided with a silver alloy according to thepresent invention as a reflective layer cleared the specifications evenafter the humidification, and each had a long-term stability. Theevaluations carried out by producing these media coincided with thealready performed, simulated test, and the long-term stability was ownedby each of the recording media, anticipated to have satisfactoryproperties in the simulated test, each provided with a reflective filmcontaining either dysprosium or gallium. Consequently, it has beenverified that the simulated test carried out in the present embodimentis a convenient method for assessing the properties of a silver alloywithout actually producing a recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the results (jitter values) of an acceleratedenvironment test carried out on DVD-R media each provided with one ofthe reflective films according to present Embodiment;

FIG. 2 is a graph showing the results (PI8 error values) of theaccelerated environment test carried out on the DVD-R media eachprovided with one of the reflective films according to presentEmbodiment; and

FIG. 3 is a graph showing the results (PO error values) of theaccelerated environment test carried out on the DVD-R media eachprovided with one of the reflective films according to presentEmbodiment.

1. A silver alloy for use in a reflective film, comprising silver as amain element and at least one rare-earth element as a first dopantelement.
 2. The silver alloy for use in a reflective film according toclaim 1, wherein the first dopant element comprises at least one ofdysprosium and thulium.
 3. The silver alloy for use in a reflective filmaccording to claim 1, wherein the first dopant element comprises atleast one of terbium, gadolinium, erbium, neodymium, holmium,praseodymium, samarium, lanthanum, cerium, ytterbium, and europium. 4.The silver alloy for use in a reflective film according to claim 1,comprising gallium as a second dopant element.
 5. The silver alloy foruse in a reflective film according to claim 1, comprising as a seconddopant element at least one of platinum and palladium.
 6. The silveralloy for use in a reflective film according to claim 1, comprising as asecond dopant element at least one element selected from magnesium,zinc, nickel, molybdenum, gold and aluminum.
 7. The silver alloy for usein a reflective film according to claim 1, comprising as a second dopantelement at least one element selected from copper, cobalt, tin,titanium, bismuth, manganese, scandium, and yttrium.
 8. The silver alloyfor use in a reflective film according to claim 1, comprising as asecond dopant element at least one element selected from silicon,chromium, iron, zirconium, niobium, tantalum, tungsten, rhodium,iridium, indium, lead, calcium, antimony, strontium, hafnium, germanium,and boron.
 9. The silver alloy for use in a reflective film according toclaim 1 comprising a second dopant element, wherein a total of theconcentration of the first dopant element and the concentration of thesecond dopant element are 0.01 to 5.0 atomic %.
 10. The silver alloy foruse in a reflective film according to claim 9, wherein the total of theconcentration of the first dopant element and the concentration of thesecond dopant element are 0.01 to 3.0 atomic %.
 11. A sputtering target,comprising the silver alloy as defined in claim
 1. 12. An opticalrecording medium comprising a substrate and a silver alloy on thesubstrate which silver alloy comprises silver and at least onerare-earth element as a first dopant element.
 13. The optical recordingmedium according to claim 12 wherein the silver alloy comprises a firstdopant element comprising at least one of at least one of dysprosium andthulium.
 14. The optical recording medium according to claim 12 whereinthe silver alloy comprises a first dopant element comprising at leastone of terbium, gadolinium, erbium, neodymium, holmium, praseodymium,samarium, lanthanum, cerium, ytterbium, and europium.
 15. The opticalrecording medium according to claim 12 wherein the silver alloycomprises a second dopant element comprising gallium.
 16. The opticalrecording medium according to claim 12 wherein the silver alloycomprises a second dopant element comprising at least one elementselected from platinum and palladium.
 17. The optical recording mediumsilver according to claim 12 wherein the silver alloy comprises a seconddopant element comprising at least one element selected from magnesium,zinc, nickel, molybdenum, gold and aluminum.
 18. The optical recordingmedium according to claim 12 wherein the silver alloy comprises a seconddopant element comprising at least one element selected from copper,cobalt, tin, titanium, bismuth, manganese, scandium, and yttrium. 19.The optical recording medium according to claim 12 wherein the silveralloy comprises a second dopant element comprising at least one elementselected from silicon, chromium, iron, zirconium, niobium, tantalum,tungsten, rhodium, iridium, indium, lead, calcium, antimony, strontium,hafnium, germanium, and boron.
 20. A method for producing an opticalrecording medium which comprises forming a film of a silver alloy on asubstrate, which silver alloy comprises a first dopant element selectedfrom at least one of dysprosium, thulium, terbium, gadolinium, erbium,neodymium, holmium, praseodymium, samarium, lanthanum, cerium,ytterbium, and europium; and which silver alloy optionally furthercomprises a second dopant element selected from at least one ofplatinum, palladium, magnesium, zinc, nickel, molybdenum, gold,aluminum, copper, cobalt, tin, titanium, bismuth, manganese, scandium,yttrium, silicon, chromium, iron, zirconium, niobium, tantalum,tungsten, rhodium, iridium, indium, lead, calcium, antimony, strontium,hafnium, germanium, and boron.